Questions

Let's set the stage here:
Since having moved back in with my parents while finding a new place of my own, I've had the chore of mowing the lawn impressed upon me, which I have no issue with whatsoever

We have more recently (within the last 1 1/2-2 years) had a new drain field put in our back yard, which included a sizeable dirt mound that has created a "hill" that needs to be mowed.

Here's where the issue lies:
My father has requested of me to not mow the hill/mound with the riding mower, and instead mow it with the small push mower that we are borrowing from a family member.

I feel that this request is a bit arbitrary, due to the fact that I've used the riding mower on the mound before with no issue multiple times.

My father is in the mindset that using the riding mower on the hill will cause the transmission to slip on the riding mower, to which my thoughts are: why would they make a riding mower that wouldn't be able to go up and down hills?"

So the question remains, is his request a bit arbitrary or am I being an asshole by telling him that using the riding mower on the hill is not gonna cause any problems?

P. S. if for some crazy instance where the transmission were to go up, I would be willing to pay for the replacement. (Even though I highly doubt that it'll happen)